This invention pertains generally to radar guidance systems for guided missiles and particularly to the missile-borne receivers in radar guidance systems incorporating semiactive guidance techniques.
It is common practice in the art to provide, in a semiactive guidance system, a missile-borne radar receiver which is adapted to distinguish between radar echo signals from a target to be tracked (hereinafter referred to simply as the target) and echo signals from other illuminated objects, such as clutter, by reason of the difference in the Doppler shifts of any echo signals received. To accomplish such purpose it is necessary that the frequency of the first local oscillator in the missile-borne radar receiver be coherent with the frequency of the illuminating signal transmitted from a control radar.
In the present state of development of local oscillators it is more feasible to provide an adjusting arrangement in a missile-borne receiver than to provide a local oscillator having the requisite stability to maintain coherence with an illuminating signal from a control radar. Therefore, it is common practice in the art to provide a so-called rear receiver in a missile-borne radar receiver operating to maintain coherence between the frequency of the first local oscillator and the frequency of the illuminating signals from the control radar.
It is also common practice to use a so-called inverse receiver in combination with a rear receiver to improve the degree with which echo signals from a target and from other illuminated objects may be distinguished. Briefly, such a receiver is a heterodyne receiver with an intermediate frequency amplifier having an extremely narrow bandwidth, means for precisely setting the frequency of a first local oscillator in accordance with the Doppler shift of echo signals from a target and correction means responsive to any change in frequency of the signals out of the intermediate frequency amplifier to adjust the frequency of the first local oscillator to null any such change.
It has in the past been known to use a voltage controlled crystal oscillator, or VXCO, in the correction means mentioned above. However, because the output frequency of any VXCO used in such an application must be linearly variable over a range corresponding to the range of possible Doppler shifts to be encountered and because the output frequency of any such VXCO must be substantially unaffected by changes in ambient temperature, it is almost impossible to provide satisfactory correction means. That is to say, with presently known elements, satisfactory correction means for adjusting the frequency of the first local oscillator in known inverse receivers are very difficult and expensive to implement.